U.S. Pat. No. 3,605,054 discloses an encapsulated solenoid which marked a substantial improvement over prior art solenoids by eliminating a number of parts and assembly operations, improved heat transfer to the ambient atmosphere, and the resin block encapsulating the parts was of sufficient structural integrity so that the solenoid could be secured directly to the device to be actuated by securing the block itself to such device. While such solenoid has been enjoying substantial commercial success, further attention has been given to reducing manufacturing and assembly costs.
It has long been an object in the design of solenoids to improve mechanical reliability and reduce manufacturing costs. To reduce eddy current losses it is common practice in industrial solenoids to fabricate the field frame (or stack) and plunger from a series of steel laminations riveted together. A typical solenoid of this construction is shown in U.S. Pat. No. 2,466,592. Such solenoids, though quite efficient, necessitate providing a series of laminations which must be riveted together and this raises the manufacturing cost over parts that are non-laminated. However, solid (non-laminated) parts increase current losses and therefore the problem is to arrive at a design which will minimize such losses at the lowest possible manufacturing cost.
One approach to eliminating the riveted laminations of the field stack is shown in U.S. Pat. No. 2,629,766 wherein the field stack comprises a spirally wound strip of steel having a central opening in which is inserted a coil. Being wound upon themselves the convolutions of the stack need not be riveted together. There is a metal tube within which the plunger reciprocates. The metal tube increases the air gap and reduces solenoid efficiency as well as contributing to unit cost.
In U.S. Pat. No. 3,633,139 an arrangement generally similar to U.S. Pat. No. 2,629,766 is shown but the solenoid coil and spirally wound stack (or frame) are encapsulated in a resin block. Here a solid plunger is utilized and a longitudinal slot is provided therein for reducing eddy current losses. In this patent as in U.S. Pat. No. 2,629,766 a metal plunger tube extends completely through the structure and this increases the air gap and reduces efficiency as well as increasing unit costs.